1. Field of the Invention
The present invention concerns recording layer compositions for use in optical recording media, and more particularly, compositions for use in recordable compact disc (CD-R) media and recordable digital video disc (DVD-R) optical media wherein the dye compositions have selected anions for improved solution stability and solubility in preferred coating solvents. The present invention also concerns selected solvents suitable for dissolving and spin-coating the dye components and, optionally, other additives such as stabilizers used for producing the recording layer, as well as substrates coated with such recording layer compositions, and recording systems implementing such recording layer compositions on a recordable medium.
2. Brief Description of Art
Read-Only Memory (ROM) type recording media have been implemented in a variety of fields such as audio recording and information processing. ROM type recording media contain previously recorded information and generally comprise prepits molded into a plastic substrate, a reflective layer comprising a metal such as gold, silver, copper, or aluminum, and a protective layer. ROM recording media are generally offered in the form of compact discs (CDs) which are available for use in audio or computer CD players. However, one drawback to this technology is that these types of recording media have no writable recording layer. This drawback has limited the usefulness of optical storage media.
A strong interest in the development of writable optical recording media has led to several technologies in the optical recording media art. In the so-called "Write Once Read Many" (WORM) recording media, information is recorded in the form of a pit "burned in" by an incident laser beam. Transient temperatures of &gt;1000.degree. C. cause ablation of the recording film and form the resulting pit. Because of the high temperatures attained during the recording process, many different materials including metal films, organic dye films and mixtures of dyes and polymer films may be used as the recording layer. Reproduction of the information stored on the disc is accomplished by irradiating the disc with a laser beam having a weaker output than that for recording. The contrast between the pitted areas and the nonpitted areas on the disc is read as an electrical signal.
CD-Recordable (CD-R) media have also been implemented as an alternative to the WORM technology. Because of their compatibility with existing read-only CD-ROM audio and computer equipment, CD-R media have been favored for many commercial applications. CD-R technology utilizes a layer of organic dye positioned between a substrate, and a reflective layer. In CD-R, the transient temperature rise resulting from absorption of radiation by the incident laser beam is less than that of WORM technology (typically 200-300.degree. C.). Moreover, the resulting dye decomposition in combination with thermal diffusion of the dye and the substrate, as well as deformation of the reflecting layer, causes local changes in the optical properties that are necessary for achieving adequate signal-to-noise ratio (SNR).
In CD-R recording, the laser wavelength is typically in the range of 770-830 nm and the recorded spot size is about 0.80 .mu.m. A significant improvement in the density of data which can be stored on a disc of the same area may be achieved by reducing the laser wavelength and hence the recorded spot size. In DVD-R recording, the laser wavelength is 630-650 nm and the spot size is about 0.40 .mu.m. Thus, significant improvements in data density are theoretically possible with DVD-R recordings over CD-R recordings. However, in practice, the wavelengths suitable for these recording and reproduction of optical data storage applications are limited by the availability of reliable, compact lasers and suitable dyes. This is especially true for consumer applications where the lasers must also be low cost, and hence have generally been of the diode type which have a limited selection of output wavelengths. For this reason, recording media must be capable of having adjustments made to the optical properties such as absorbance and reflectivity in order to tune their response to an available laser wavelength.
The dye recording layer of the optical recording medium is formed on a pre-grooved substrate, generally comprising a injection molded disc of polycarbonate. The grooves are present to assist the guiding of the laser beam both on recording and upon subsequent reproduction of the recorded signal. The dye recording layer is deposited by spin-coating from a solution of the dye components in such a manner as to produce a uniform thickness deposited both within the grooves and on the adjacent lands between the grooves. For a typical optical recording medium, the average film thickness of the dye layer over the lands and grooves of the substrate is in the range of 50-160 nm, and for CD-R is typically in the range of 70-130 nm.
The organic dyes useful for optical data applications such as CD-R and DVD-R generally include classes of compounds in which the optical absorption characteristics may be manipulated to achieve the best overall combination of functional characteristics, such as write sensitivity, read stability, solubility in coating solvent, shelf life, and other properties known to those skilled in the art. Such organic dyes also offer low thermal conductivity which results in low heat loss and higher writing sensitivity (reduced laser power) compared with other highly absorbing materials such as metal films. The preferred classes of organic dyes used in the past for many optical data recordings include cyanine dyes, particularly indodicarbocyanines (also referred to as INCY), benzindodicarbocyanines (also referred to as BINCY) and the hybrid indodicarbocyanine- benzindodicarbocyanines (also referred to as INCY-BINCY). Structures of particular examples of each of these classes of organic dyes are shown in formulae (I) to (III), respectively: ##STR1## wherein in each formula, X.sup.- is a selected counteranion, n is either 1, 2 or 3 and R.sub.1 and R.sub.2 are alkyl groups having 1-4 carbon atoms for formula (I) and alkyl groups having 1-5 carbon atoms for formulae (II) and (III).
The wavelength region of maximum light absorption in any particular cyanine dye (or the intensity of absorption at a given wavelength) depends on the structure of the organic cation and in particular on the length of the methine chain which joins the indole or benzindole moiety and is defined by n in the above formulae. Thus, by varying n, the wavelength sensitivity to a particular recording laser wavelength may be altered. For CD-R media, having a recording wavelength in the 770-790 nm range, n is typically 2. For DVD-R media having a recording laser wavelength in the range of 630-650 nm, n is typically 1, and for WORM media, for which a higher absorption at the same recording wavelengths is required compared with CD-R or DVD-R media, n is either 2 or 3. In compositions where it is advantageous to finely adjust the optical properties of the dye layer, a mixture of dye components is often preferred, in which each component may have a different value of n. Other advantages of these classes of cyanine dyes in optical data recording applications are the relative ease of synthesis and the acceptable thermal and photochemical stability both in solution and in the solid film.
In the past, the above classes of cyanine dyes employed as an anion (X.sup.-) either a halide (preferably iodide), a perchlorate, a sulfonate, a toluenesulfonate or a tetrafluoroborate. The iodide and perchlorate counteranions were the most commonly used. The dyes containing these counteranions were chosen for their ease of synthesis and purification.
The majority of patents related to the organic dyes used in CD-R are directed to dye compositions having certain improvements with regard to (1) solubility in coating solvents (also called casting solvents) such as cellosolve, methyl cellosolve, ethyl cellosolve acetate, methylcarbitol, diacetone alcohol, or mixtures thereof with alcohols such as amyl alcohol, heptanol or cyclohexanol; (2) stability against thermal oxidization and photooxidation; (3) reduced dependence of the recording characteristics on wavelength of the laser light (especially in the range 775-830 nm which characterizes the range of wavelengths emitted from GaAs lasers under typical operating conditions); (4) higher recording sensitivity; and (5) higher recording and read-out stability. Typical past improvements include combinations of one or more dyes with stabilizers, often referred to as quenchers. Particularly suitable quenchers which reduce photooxidation have generally been based on transition metal complexes especially those of Ni and Co which may be coordinated to sulfur atoms, and certain types of aromatic tertiary amines capable of forming stable radicals, which are optionally combined with a Ni or Co dithiolato type complex. In U.S. patent application Ser. No. 08/746,449 filed on Nov. 8, 1996 it was shown that the use of a stabilizing dye molecule especially of the amine type, which possesses strong optical absorbance in a wavelength region above the recording wavelength, provides an additional advantage in reducing the wavelength dependence of the dye recording layer.
Also in the past, dye molecules have been modified to improve solubility in spin-coating solvents which do not dissolve the polycarbonate disc substrate material which is widely used for both CD-R and DVD-R media. The number of suitable casting solvents is limited by the number of constraints such as high dye solubility, low rate of attack or swelling of the polycarbonate substrate, relatively high boiling point and medium-fast evaporation rate to achieve uniform coating thickness and high throughput, low toxicity and low cost. Solvents which have met several of these criteria are ethyl cellosolve acetate, diacetone alcohol and tetrafluoropropanol and mixtures often with lower aliphatic alcohols. However, each of these solvents suffers from one or more disadvantage. Ethyl cellosolve acetate, is a member of the family of ethylene glycolethers known to posses an undesirable level of toxicity which may adversely effect mammalian reproductive behavior. Tetrafluoropropanol is expensive, has a limited range of solvency and has a relatively high evaporation rate which makes the spin-coating process more difficult to control. Diacetone alcohol has a tendency to decompose in the presence of oxygen forming peroxides, and can undergo anaerobic decomposition in the presence of other species capable of oxidation. Such reactions reduce the chemical stability of a dye solution comprising diacetone alcohol as the main solvent. Although diacetone alcohol does not dissolve polycarbonate to any significant extent, its solvency is high enough to attack the surface of the disc during spin-coating. Although this improves the adhesion of the dye layer, an excessive amount of attack reduces the integrity the pre-groove features and causes a reduction in the quality and consistency of the recording and reproducing characteristics of the disc. Furthermore, the introduction of traces of polycarbonate and the relatively poor chemical stability of the dye components in the solvent can limit the ability to reuse the portion of solution which is spun off the disc during coating, and restrict the possibility of achieving economical use of the dye. Separately, ethyl lactate is a known safer solvent from a toxicological standpoint, has the desirable physical properties for spin-coating, possesses excellent chemical stability and has other unexpected advantages which are explained below.
Modification of cyanine dyes is generally accomplished by the introduction of solvent compatible substituents onto selected atoms of the parent rings or in some cases the carbocyanine chain (also known as a polymethine chain). These substituents may be alkyl, alkoxy, alkoxyether, halogen, or combinations of substituents. In the case of indocarbocyanine and benzindocarbocyanine dyes, increasing the solubility in selected solvents is usually accomplished by substitution of a moderately hydrophobic lower alkyl group, such as propyl, butyl, or pentyl onto the nitrogen atom of the indole ring. Attempts to improve the solubility in coating solvents by changing the anion, typically to an organic species such as an acetate or toluenesulfonate have also been made. However, in most cases, the increase in solubility is relatively small and the desired solubility in coating solvents could only be obtained by suitable modification of the cyanine dye cation.
Moreover, solutions of cyanine dyes in coating solvents often have a tendency to recrystallize on storage or upon temperature cycling, especially to a temperature range below room temperature, such as upon refrigeration, which is desirable to increase the chemical stability of the solution. Although this problem may be reduced in severity when an asymmetrical cyanine dye of the type shown in formula (III) above is chosen, the problem becomes more severe for solutions having the relatively high concentrations of dye components needed to achieve desirable coating characteristics, and in the presence of relatively large concentrations of stabilizers required to achieve desirable photostability and recording sensitivity.
Thus, the practical use of cyanine dyes for optical data storage media depends upon a large number of factors which influence the selection of dyes, the solvents and added components such as stabilizers. The practical use can be extended by consideration of the dye composition from the following aspects: 1) compatibility with the recording wavelengths, 2) solubility, 3) substrate compatibility, 4) solution stability, 5) safety and 6) cost.
For example, U.S. Pat. No. 5,328,741, assigned to Pioneer Electronic Corp., discloses mixtures of cyanine dyes. In one embodiment, the dyes are a mixture of an indodicarbocyanine-type in which n=2, having a propyl groups attached to both of the indole nitrogen atoms, and a benzindodicarbocyanine-type dye in which n=2, having butyl groups attached to both of the indole nitrogen atoms. The patent further teaches that these specific dyes are readily soluble in a good coating solvent such as diacetone alcohol (DAA). This patent also discloses the use of a specific metal quencher. The disclosed anions for these cyanine dye mixtures are perchlorate, bromide and iodide.
U.S. Pat. No. 5,336,584, assigned to Pioneer Electronic Corp., discloses a composition containing benzindocicarbocyanine-type dye and a nickel tetrathioate quencher. The disclosed anion is perchlorate and the solvent is 2-ethoxyethanol.
U.S. Pat. No. 5,328,802, assigned to Pioneer Electronic Corp., discloses a particular benzindodicarbocyanine-type dye in combination with a nickel tetrathiolate quencher. The disclosed anion is perchlorate.
U.S. Pat. No. 5,155,009, assigned to Pioneer Electronic Corp., discloses a particular indotricarbocyanine-type dye in combination with a nickel tetrathiolate quencher. The disclosed anion is perchlorate and the disclosed solvents are 2-ethoxyethanol, 2-methoxyethanol and cellosolve acetate.
U.S. Pat. No. 5,161,150, assigned to TDK Corp., discloses the use of a dye mixture comprised of different tricarbocyanine and dicarbocyanine dyes. The disclosed anions for these cyanine dyes include perchlorate and toluenesulfonate.
U.S. Pat. No. 5,512,416 assigned to TDK Corp., discloses the use of a recording layer comprising at least one substituted indolene type carbocyanine dye in which the substituents on the indole nitrogen atoms each contain more than four carbon atoms and the methine chain has n=1 to 4. The disclosed anions for these cyanine dyes include perchlorate, tetrafluoroborate, bromide and iodide.
U.S. Pat. No. 5,161,150, assigned to TDK Corp., discloses the use of a dye mixture comprised of different tricarbocyanine and dicarbocyanine dyes. The disclosed anions for these cyanine dyes include perchlorate and toluenesulfonate.
U.S. Pat. No. 5,213,955, assigned to Taiyo Yuden Co., Ltd., discloses the use of a thick recording dye layer for CD-R comprising at least one substituted indolenedicarbocyanine dye. The disclosed anions for these cyanine dyes include halide, perchlorate, fluoroborate, benzenesulfonate, toluenesulfonate, alkylsulfonate, benzenecarboxylate, alkylcarboxylate and trifluoromethylcarboxylate. The claim in this patent is limited to a specific range of average film thickness estimated to be between 180 and 480 nm and outside the normal range used for the production CD-R and DVD-R media.
U.S. Pat. No. 5,316,184, assigned to Fuji Photo Film, discloses the use of substituted indolene type carbocyanine dyes having a hexafluorophosphate anion.
U.S. Pat. No. 4,600,625, assigned to Ricoh Company Ltd., discloses the use of a thin film recording layer free of a metal reflecting layer containing a substituted indolenecarbocyanine dye in which the methine chain has n=1-3. The disclosed anions for these cyanine dyes include halide, perhalogenate, tetrafluoroborate, benzenesulfonate, toluenesulfonate, alkylsulfonate. The disclosed solvents for these cyanine dyes are dichloroethane and ethyl alcohol, the former being used on a glass substrate only.
U.S. Pat. No. 5,316,814, assigned to Ricoh Company Ltd., discloses the use of solvent mixtures of tetrafluoropropanol with 2-methoxyethanol for casting a benzindodicarbocyanine-type dye.
U.S. Pat. No. 4,944,967, assigned to Fuji Photo Film, discloses the use of diacetone alcohol (DAA) as a casting solvent.
Despite these modifications, however, the solubility and/or the solution stability of the dye formulations in preferred coating solvents such as DAA and especially ethyl lactate may be inadequate to achieve the higher concentrations (typically &gt;8 wt % of solids in solution) desirable for high coating uniformity and reduced cycle time for an efficient coating process. Furthermore, the stability of the solutions may be too poor to permit economical recycling of the dye which is not consumed during the spin coating process.
An advantage of the present invention is the improved solubility and greater solution stability of cyanine dyes and formulations of cyanine dyes with combinations of stabilizers, which makes possible the use of a coating solvent such as ethyl lactate having excellent film forming characteristics, reduced toxicity and high solution stability, as well as excellent compatibility with a polycarbonate substrate. A further advantage of the present invention is that other solvents such as DAA, ethyl lactate and various blends of these solvents with alcohols and buytl lactate for example, may also be used, in which sufficiently high dye concentrations can be achieved to reduce the cycle time of the spin coating process and achieve excellent uniformity of the coated film across the whole area of the substrate upon which the dye layer is deposited by spin coating. These advantages are achieved by combining a cyanine dye cation with selected anions, each of whose sodium salt exhibits good solubility in a lower alkyl alcohol.